The present invention relates to an method and apparatus for the highly accurate characterization of radiation fields.
The evaluation of radiation fields is indispensable in many areas, as, for example, in antenna near-field measuring technology. In near-field measuring, which is preferably used for antennas in the frequency range from approximately 0.5 to 20 GHz, the immediate electromagnetic near field of an antenna is measured and is converted by means of a near-field (NF) to far-field (FF) transformation into the far field by means of the Fast Fourier Transformation (FFT). The advantage of measuring the near field of an antenna lies in the compact dimensions of the necessary antenna measuring systems, which heretofore have almost exclusively been integrated into stationary measuring chambers.
In contrast to near-field measuring systems, there are also far-field measuring systems. However, due to their dimensions, these are exterior systems, and are always stationary devices. also, they are considerably more prone to error as a result of reflections from the environment, terrain formations, buildings, etc.
Another advantage of the near-field measuring technique is that, as a result of a near-field recording, all far-field sections can be computed, while the once measured far-field sections are fixed and the antenna has to be measured again for additional far-field sections at a later point in time.
In accordance with the scanning theorem, the near field is scanned in  less than xcex/2 intervals, and the entirety of the electromagnetic radiation emitted by the antenna must be detected, down to approximately xe2x88x9245 db, because the totality of these measuring points has an influence on each individual computed far-field point.
For measuring the radiation fields of omnidirectional antennas, spherical scanners are usually used, which scan the near field of the antenna to be measured on a spherical surface. In the case of directional antennas, the high-expenditure spherical scanners may be eliminated, as long as all radiation fractions down to approximately xe2x88x9245 db can be detected on a cylinder surface or on a planar surface. Since directional antennas (parabolic antennas) are mainly used, for example, in telecommunications, the selection in this field usually leads to cylindrical near-field measuring systems or planar systems.
In the NF to FF transformation, in addition to the amplitude values of the individual measuring points, phase information is also used. Therefore, a scanner, , should be able to scan a spherical surface, a cylinder or a planar surface by means of a measuring probe as nearly ideally as possible, because the NF to FF transformation is mathematically based on this ideal case. Error contributions by the scanner of a near-field measuring system should not exceed a deviation of xcex/50 from the ideal contour.
Thus a scanner accuracy of 3.0 mm, at f=2.0 GHx and a phase accuracy of xcex/50 are necessary. If ground station antennas with an antenna diameter of, for example, 14 mm are to be measured by means of a planar measuring system, this degree of accuracy must be achieved on a surface of at least 20 mxc3x9720 m.
For use with radar systems, near-field scanners should be as invisible as possible. This is of course contrary to the normal mechanical structures required for such scanners, and as a rule can be achieved only by the use of corresponding absorber coverings.
In order to obtain a maximum of phase accuracy of the measurement, data recording should be recorded for of all measuring points as rapidly as possible in order to minimize temporal phase drifts as much as possible.
Based on the above-mentioned example, with a surface to be scanned of 20 mxc3x9720 m and a measuring point distance of 75 mm, an array of 267 measuring points in width and 267 measuring points in height of the antenna, results in a total of at least 71,289 measuring points. A rough estimate shows that it would require unacceptable expenditures to drive to each of the measuring points, so that measuring must take place during the drive while passing the measuring position. At a scanning speed of 100 mm/sec., data recording would therefore require approximately 15 hours.
From Stehle et al., xe2x80x9cReledop: A Full-Scale Antenna Pattern Measurementxe2x80x9d L.E.E.E. Trans. On Broadcasting, Volume 34, No. 2, June 1988 (1988/06, Pages 210-220 YP 000054225 New York, US) and also Henxcex2, xe2x80x9cHubschrauber-Messungxe2x80x9d NTZ Nachrichtentechnische Zeitschrift, Volum 40, No. 4, April 1987 (1987/04, Pages 258-261, YP-002168218) Berlin, Del.), it is known to arrange probes by means of a pilot-controlled helicopter with the interposition of a long trail rope or a telescopic rod in a field to be measured. The use of a real helicopter and the interposition of long trail ropes or telescopic rods, however, do no permit highly accurate measuring, and particularly no highly accurate positioning within the field to be measured.
It is an object of the present invention to provide an method and apparatus for a highly accurate evaluation of radiation fields, by means of which highly accurate and large-surface measurements of radiation fields can be carried out at relatively low expenditures, particularly in the exterior region.
This an other objects and advantages are achieved by the measuring arrangement (particularly a mobile measuring arrangement) for the alignment/position and/or detection of electromagnetic characteristics of devices for/with the] emission of radiation fields according to the invention, which includes a remotely-controllable measuring device that can hover, and has a measuring probe for detecting the targeted signal, as well as at least one device for determining the attitude and position of the measuring device.
For determination of the attitude and position, position determination systems are preferably arranged in the vicinity of the emission device, in the form of position receivers/antennas that are provided in a defined position relative to the hovering device.
In the measuring device according to the present invention, preferably a highly accurate global, non-terrestrial position determination system (such as the GPS) is used as the position determination system.
Furthermore, it is preferred that the position receiver/antenna of the system for measuring the site, the position and the attitude, is arranged on the measuring probe. In order that the electromagnetic measurement conform as accurately as possible to the position determination or alignment of the emitting device, the phase center of the measuring probe should be situated as close as possible to the position receiver/antenna.
Furthermore, the emission device is preferably an antenna and, more specifically, a parabolic antenna or an array antenna.
In addition, the measuring arrangement may be include a combination of the position receiver/antenna, a compass, a device for measuring inertia forces, and one or more rotation sensors for determining and controlling the attitude of the hovering device. To the extent that it may be necessary in a special application, other components can be added.
According to another feature of the measuring device has a plurality of spatially separated position receivers/antennas. This permits the use of a differential method for determining the position and attitude of the hovering device.
In a further embodiment of the measuring arrangement according to the invention, an additional position receiver/antenna is provided as a reference on the ground in the area of the emission device. This permits the use of a differential method for determining the position and attitude of the hovering device.
In a measuring arrangement constructed in this manner, direct visual contact is not required between a ground station (at which, for example, the measuring equipment for processing the data supplied by the measuring probe, as well as devices for controlling the hovering measuring device can be provided) and the receiver. This may be an advantage, particularly in the case of spherical scanning contours.
The position receivers/antennas and/or the measuring probe on the hovering device can advantageously be arranged in such a manner that angular adjustment, swivelling or stabilization of the measuring probe is possible (in order, for example, to ensure a correct alignment, independent of an inclined position of the hovering device such as a helicopter, even under the effect of wind.) In particular, stabilization for small position and angle deflections can be provided which, taking the relative position of the emission device. This stabilization and/or positioning can advantageously also interact with the measuring control circuit, so that a corresponding tracking can be displayed. As a result, a tolerances can be compensated, and therefore the individual measurements can be accelerated.
In another embodiment of the measuring arrangement according to the invention, devices may be provided on the measuring probe for detecting the signal, hovering in front of the emission device. The relative momentary measuring position of these devices is detectable by at least one geodetic instrument which is equipped with a device for emitting a defined optical signal, a device for receiving an optical signal, and a device for reflecting the defined optical signal of the geodetic instrument at the position to be measured. The reflecting device may be, for example, a spherical reflection surface, so that the reflection of the defined optical signal is reduced to a point for the viewer, and/or spherical reflection surfaces may be provided in a defined relative position with respect to the hovering device and/or the measuring probe.
The reflection surface may be part of a metal-coated sphere.
According to a further embodiment of the invention, the geodetic device for receiving an optical signal may be provided with a concave primary mirror, a convex secondary mirror and a detector device sensitive in two dimensions (such as a position diode) for generating a reading signal. As an alternative to the mirrors, other optical systems, such a reflectors/refractors can also be used.
According to yet another embodiment of the invention, the secondary mirror may be placed essentially in the focus of the primary mirror, with the detector device placed opposite the secondary mirror in the area of the primary mirror, preferably behind an opening in the primary mirror, through which the reflected optical signal passes which is focussed in the secondary mirror.
Likewise, two geodetic instruments are preferably assigned to each reflection device, so that a cross bearing is permitted.
The optical signal emitted by the geodetic instrument is preferably a laser beam, particularly a power-adjustable and/or modulable laser beam, and is provided with highly accurate angle-position encoders in the azimuth and in the elevation, for the dynamically accurate detection of the bearing angles with respect to the respective reflector. For example, when two laser beams are used, they can be modulated with a different frequency, permitting identification of the reflected signal. Also, in a particularly preferred embodiment, the power adjusting capability is provided as a function of the distance between the laser source reflector and the detector device. In this manner damage to the diode due to excessive laser irradiation can be avoided. It was found to be particularly advantageous to use a semiconductor laser as the laser beam, so that modulation can be represented as an alternative or in a supporting manner also by frequency filters.
In this case, this measuring arrangement is preferably constructed such that three of the above-mentioned arrangements are provided, with three reflection surfaces in a defined relative position on the hovering device.
The measuring arrangement itself can detect electromagnetic characteristics in a manner that is known per se. Normally a measuring probe is used for this purpose. Thus, a reciprocal relationship can be achieved between the electromagnetic measurement, the measuring site and/or the position of the radiating device. As a result of the highly accurate relative determination of the three parametersxe2x80x94position, field and generating of the fieldxe2x80x94, it is possible in a simple manner to carry out a plurality of highly accurate measurements, in which case the measuring probe can be operated, for example, by using the initially described near-field measuring technique.
Furthermore, one of the spherical reflection surfaces is preferably arranged on the measuring probe. In order to maximize the degree of conformity between the electromechanical measurement and the position determination or alignment of the radiating device, the phase center of the measuring probe should be situated as close as possible to the center point of the spherical reflection surface. Optimum precision is obtained when the center point of the sphere and the phase center coincide. In addition, the emission device preferably is an antenna and, more specifically, a parabolic antenna or an array antenna.
In addition to the above-mentioned characteristics, the measuring arrangement according to the invention may include an autofocussing device for imaging the reflected laser beam, which speeds detection of individual measuring points, and increases their precision. It should also be mentioned that also the relative position of the diode or the detector device can be evaluated in the display area in order to further increase the measuring accuracy.
The size and the mass of the hovering device is preferably small in relationship to that of the emission device that is to be positioned, because objects in an electromagnetic field to be measured may result in considerable measuring errors. In order to meet this requirement, it is advantageous to provide, for example, a miniature helicopter as a hovering device. However, other alternatives, such as controlled balloons, zeppelins, or similar devices, are also conceivable, which preferably are radio-controlled.
In addition to the measuring, the invention also provides a method for the highly accurate evaluation of radiation fields, particularly for mobile use and/or in the exterior region. The method advantages according to the invention comprises the following steps:
1. positioning a hovering remotely-controllable measuring device in the radiation field, with a measuring probe for the detection of the radiation field at least one device for determining the attitude and position of the measuring device;
2. determining the position and attitude of the measuring device; and
3. generating a measuring signal for characterizing the radiation field; and
4. transmitting of the measuring signal from the hovering part of the measuring arrangement to a ground-side measuring instrument system.
According to the invention, the method can be further developed such that the coordinates of the systems can be determined in three spacial dimensions, and from these coordinates, position and the actual attitude of all six degrees of freedom of the measuring device are dynamically determined (particularly in real time).
Furthermore, the actual position and attitude (all six degrees of freedom) of the measuring device can be compared with the defined desired position and attitude, and can be controlled in a closed-loop control circuit during the controlling, stabilization or positioning of the measuring probe.
Finally, a person skilled in the art will understand that, although the present application addresses a radiating device, the invention can also be used in a reversal/supplementation in the case of a receiving system or a field-alternating, particularly a reflecting device.
One decisive advantage of the measuring arrangement according to the invention is that its mobility permits a complete and highly accurate characterization of radiation characteristics of large, usually stationary antenna systems in the exterior region.
Additional advantages of the invention include:
a high positioning precision from approximately 2.0 mm to 50 m;
large positioning ranges of up to 100 m edge length of a cube;
high positioning speed  less than 1.0 min over a positioning route of 10 m;
highly accurate detection of all 6 degrees of freedom of 0.5 mm and 1.0 angular minutes at a distance of 50 m;
mobility;
lower installation expenditures; and
broad application spectrum (antenna measurements, radar backscattering measurements, electromagnetic compatibility measurements, environmental measurements, etc.)